This invention relates to vertical form, fill and seal machines, and in particular to a method and apparatus for protecting the back seal formed by the vertical form, fill and seal machine as product to be packaged is inserted.
For years, vertical form, fill and seal machines have been used to package various products by forming plastic film around a forming tube, making a back seal in plastic film while it is on the forming tube in order to form the plastic into a generally cylindrical shape, and then inserting product to be packaged by dropping it down the forming tube into partially formed packages before they are finally sealed. Once product is inserted, subsequent operations occur to add end seals which complete the package. Such an apparatus is disclosed in U.S. Pat. No. 4,288,965, the disclosure of which is incorporated herein by reference.
In the typical vertical form, fill and seal machine, when product is inserted down the forming tube, the plastic package is already sealed at its bottom end. Thus, as products fall within the forming tube, some back air pressure is built as products fall down the forming tube, such that back pressure being exerted within the partially-formed package against both the lateral bottom seal in the package and the longitudinal back seal for the package.
When small products are packaged, creation of back pressure is not a problem, since displaced air simply exhausts up the forming tube around the product as it is being dropped into the partially-formed package. However, when the size of the product increases, the back pressure increases and problems can occur. When a large product that nearly equals the diameter of the forming tube is inserted, the product acts like a piston with gravity drawing the product down the forming tube and air in the forming tube being compressed as the product moves downwardly. This can create two detrimental effects. First, the creation of back pressure can retard the velocity of the product, thus limiting the through put of the vertical form, fill and seal machine. Second, increased pressure in the partially-formed package can place excessive strain on both the lateral end seal and the longitudinal back seal of the partially-formed package.
The creation of back pressure is much like the action of a typical shock absorber. The greater the amount of air that can bypass the falling product, the faster the product will fall through the forming tube. Thus, the shape and size of the product being inserted has a considerable affect on the time that it takes the product to drop into the partially-formed package. If the time that the product takes to drop into the partially-formed package varies, this can result in product being trapped in either the final end seal being formed or a stager, if a stager is used to accommodate the weight of the product being inserted. Variability is particularly difficult to control, and product jams are a very undesirable result.
Various means have been developed in the past to try to relieve undesirable back pressure. One method previously utilized was to add vent tubes to the inside of the forming tube in order to exhaust air more efficiently than to simply rely on exhausting air bypassing any gaps between the product and the forming tube. Another possible way to alleviate the problem is to add vent holes to the plastic film, but this typically is unacceptable when a fully sealed package is desired.
Back pressure is not an entirely undesirable effect, since it cushions a product as it drops down the forming tube to help ensure less impact when entering the partially-formed package or impinging upon a stager which is utilized to absorb the impact of the descending product. However, back pressure is undesirable if it affects the quality of either the lateral end seal or the longitudinal back seal of the package.
Generally, when a heavy product, such as a bag filled with liquid, is being inserted down the forming tube, a stager is utilized to absorb the energy of the falling product. Utilization of a stager also greatly reduces the affect of back pressure on the lateral end seal, which is located below the stager. Also, in general, the integrity of the end seal is not a serious concern since typically the sealing pressure for the lateral end seal is higher thus allowing a lower sealing temperature and increased dwell time for the sealing jaws as compared to the platen with lower pressure, higher sealing temperature, and shortened dwell time used for the longitudinal back seal. Also, with the stager protecting the end seal, the back seal is typically the area where problems of integrity of the seal occur. Protecting the back seal is the primary object of the present invention.